Many medical diagnostic tests are performed in a medical laboratory, on serum and plasma. Serum is the yellow liquid obtained from blood after the blood is allowed to clot, and the clot is removed by centrifugation; plasma is the yellow liquid obtained from blood by centrifugation of blood mixed with an anticoagulant, e.g. heparin. Whole blood comprises the formed elements (i.e., the cellular components and the cell-derived components), and plasma. Red blood cells are the most abundant formed elements in blood, and platelets are examples of cell-derived components. In the case of a blood sample, the filtrate usually refers to the blood plasma (or simply, plasma), depending on the size of the pores in the porous membrane, and the retentate or concentrate usually refers to blood with enriched concentration of cellular components.
For Point-of-Care Testing (POCT), whole blood is usually used because the sample does not have to be processed before testing. If serum and plasma were as easily available as whole blood, the serum and plasma would be preferred because they are not as complex as blood, and therefore serum and plasma would produce more accurate test results. Plasma is usually preferred over serum because the blood needs to sit at room temperature for about half an hour in order to complete the clotting process, and then the serum is extracted by centrifugation of the sample; blood can be centrifuged immediately after the blood is collected in a tube containing an appropriate anticoagulant, in order to extract the plasma from the blood.
The inventor was awarded U.S. Pat. Nos. 7,816,124 and 7,807,450 that disclose cartridges for rapidly extracting plasma from blood. These cartridges can be used to virtually collect plasma directly from a patient's blood vessel or from some other blood supply. The cartridges use negative pressure created by manually compressing flexible members of compression chambers, for creating blood flow, and for pulling plasma from whole blood across a membrane. U.S. Pat. Nos. 7,816,124 and 7,807,450 describe the use of the thumb and forefinger for compressing flexible members of blood and plasma compression chambers. U.S. Pat. Nos. 7,816,124 and 7,807,450 also described flexible members that possess physical properties that will allow the flexible members to rebound at desired speed. Nevertheless, there is still a need for an apparatus that extracts plasma from blood in a simpler and more efficient manner. Particularly, there is a need for a cartridge that can be used instead of an evacuated tube, whereby no centrifugation has to be performed in order to extract the plasma.
There is also a need to extract an ultra-filtrate from a sample, for example serum or plasma, without the need for centrifugation. Many therapeutic drugs, ions and hormones are highly protein bound. Only the free therapeutic drugs, ions and hormones are available to cross vascular walls and biological membranes in order to interact with biologically important binding sites. Some examples of a therapeutic drug, an ion and hormone are phenytoin, calcium and thyroid hormones (T3 and T4) respectively.
Phenytoin, for example, is a therapeutic drug used to treat epilepsy. In the blood, about 90% of the phenytoin is bound to plasma proteins. Only the portion of phenytoin that is unbound or “free” is pharmacologically or biologically active. A test for total phenytoin represents the sum of the bound and unbound phenytoin. Under normal conditions, the balance between bound and unbound phenytoin in the blood is relatively stable, so measuring the total phenytoin is appropriate for monitoring therapeutic levels of phenytoin. However, in certain conditions and disease states, that balance can be upset, causing the percentage of free or active phenytoin to increase. Consequently, a patient may experience symptoms of phenytoin toxicity even though the total phenytoin result falls within a therapeutic range. In such cases, doctors may order a free phenytoin test to more reliably monitor the patient's phenytoin levels, instead of a test for total phenytoin.
One method used to measure free phenytoin in a patient's serum or plasma sample involves: 1) adding the patient's sample to the sample reservoir of an ultra-filtration device; 2) capping the sample reservoir; 3) placing the ultra-filtration device in a centrifuge and centrifuging for about 25 minutes; and 4) measuring total phenytoin in the ultra-filtrate of the serum or plasma.
As in the case of extracting plasma from whole blood, there is a need to eliminate the step of centrifugation in order to measure free phenytoin. Because the size of the proteins that bind phenytoin is larger than the pore size of the membrane in the ultra-filtration device, the bound phenytoin cannot travel with the ultra-filtrate.
In the case of a serum or plasma sample, the filtrate (or more appropriately, referred to as an ultra-filtrate) usually refers to the serum or plasma containing the smaller molecular weight substances like the free phenytoin, and the retentate usually refers to serum or plasma containing the higher molecular weight substances like the proteins that bind phenytoin. An example of such a protein is albumin, having a molecular weight of about 66 kilodaltons. In contrast, the molecular weight of phenytoin is about 0.25 kilodaltons. A person of ordinary skill in the art will appreciate that an ultra-filtrate is still a filtrate, and the term ultra-filtrate is only used for clarity when the starting sample is plasma, for example, which is already considered to be a filtrate of blood.
By way of examples only, some embodiments of a cartridge or filtration assembly can be used to perform the following: extract plasma from whole blood; extract cell-free liquid from a non-blood sample comprising cells; extract a plasma ultra-filtrate containing a free drug/hormone from plasma; and concentrate the drug/hormone or cellular components of a sample. Some advantages of the embodiments of the present invention over U.S. Pat. Nos. 7,816,124 and 7,807,450 awarded to the inventor will be described.